<div v-for="(item, index) of data.slice(start, end)" :key="item.key" :class="['item', 'item-' + (item.key + 1)]" :ref="el => setItemRef(el, item)">
  <div class="title">{{ item.key + 1 }}.&nbsp;{{ item.text }}</div>
  <div v-for="child of item.children" :key="child.key" :class="['child', 'child-' + (child.key + 1)]">
    {{ child.key + 1 }}.&nbsp;{{ child.text }}
  </div>
</div>